Transmission Trouble

I have a 2000 Pontiac Grand Prix with the 3800 V6 and just over 40,000 miles. The car is driven daily but not very far as you can no doubt tell. Just recently I began to work about thirty miles from my home and after about 20 minutes driving at 55 MPH the automatic transmission begins to lurch into gear at every upshift, enough to shake the car somewhat.

If I shut the car off for just a few minutes and restart, it goes away but returns after five minutes or so. Because I have little faith in the dealer I bought it new from (long story) I took it to a local transmission shop known to be very dependable. He found a damaged channel plate and showed me the overdrive transaxle splines were nearly worn completely off. He replaced these parts along with a Sonmax (?) torque signal valve, pressure control solenoid, 4th clutch hub, gasket seals, new filter and fluid. (Please forgive any misspellings- his writing is pretty bad) That was 2 weeks ago and it ran great until today when the same symptoms returned. My question is, what did he miss or do I just have a lemon?

Take it directly back to him. It sounds like the computer has picked up a problem and is commanding full line pressure from the pressure control solenoid. He’ll probably scan it first for any trouble codes. Looks like from the parts he replaced, he covered all bases during the overhaul. Follow up with us and let us know what he finds.


Thanks for the info on full line pressure, transman, I’ll mention it to him and follow up here, I have an appointment on 8/11. As for scanning for trouble codes, he did that first thing and found nothing, that’s why he recommended opening it up. I’m wondering if it may be a computer problem since it failed to report any issues when one (or more) must exist. Thanks again.

big al9 5

Transmission trouble-Follow up.
I just heard from the shop. This problem has him stumped. He said a scan revealed an 1811 code and he cannot understand why the problem returned after two weeks of running perfectly. He wants to do some research and ask a few friends what they think before he opens it up again. We need the car for work and he said it would be alright to drive in this condition. It’ a 40 minute drive one way at speeds between 40 and 55mph. Should I drive it???


 What the P-1811 means is that the computer has detected that the maximum allowable shift time has been exceeded and all the shift adapts have reached their upper limits.  Now the computer will max out line pressure for this code, this is why you have harsh shifting.  The maximum line pressure protects your transmission.  Usually after overhaul, the trans tech is supposed to clear all shift adapts with a scanner.  What I would do if I were working on this is to first clear all shift adapts and check with the dealer to see if there are any software updates.  If the overhaul went well and the transmission is ok, this should clear up the problem.